Conventionally, as the syringe drive unit of this type, syringe drive units disclosed in JP 2006-070868 A (Patent Literature 1) and JP 2004-162560 A (Patent Literature 2) have been well known. Those syringe drive units each include a syringe holder for holding a syringe body, a plunger pressing member for pressing a plunger housed in the syringe body, and a feed screw mechanism for moving the plunger pressing member into an axial-line direction of the syringe body in accordance with rotation of a motor. When the motor is rotated, the plunger is pushed into the syringe body in accordance with a rotational amount thereof, and liquid medicine loaded into the syringe body is ejected from the syringe body in accordance with a pushing-in amount of the plunger.
In the syringe drive unit structured as described above, the liquid medicine in the syringe body is ejected in accordance with a moving amount of the plunger, and hence it is necessary to strictly control a moving amount of the plunger pressing member. The moving amount is controlled, for example, through what is called open loop control using a stepping motor as the motor, and what is called closed loop control performed while an actual moving amount of the plunger pressing member is grasped using measuring means such as a potentiometer.